Explosive primer

ABSTRACT

A priming device especially adapted for use in combination with a blasting cap to initiate relatively insensitive blasting agents in small-diameter boreholes comprising a cap-sensitive, flexible, self-supporting explosive charge; a thin-walled tubular member in fixed contact with the charge; and means for coaxially positioning a blasting cap in the tubular member so that the cap&#39;&#39;s base charge is in initiating relationship with the selfsupporting explosive charge and both ends of the cap are protected from exposure, e.g., to potentially hazardous conditions. An assembly comprising the priming device and a blasting cap positioned therein, in which assembly the leg wires or fuse, which are adapted to connect the cap to an outside energy source, are secured to the tubular member so as to prevent the relative movement of cap and tubular member, and thereby maintain the initiating relationship between the cap&#39;&#39;s base charge and the self-supporting explosive charge.

United States Patent Schaefer Jan. 29, 1974 EXPLOSIVE PRIMER [5 7] ABSTRACT [75] Inventor: gimam ag Schaefer A priming device especially adapted for use in combiagerstown nation with a blasting cap to initiate relatively insensi- [73] Assignee: E. I. du Pont de Nemours and tive blasting agents in smalLdiameter boreholes com- Company, Wilmington, Del.

Primary ExaminerVerlin R. Pendegrass prising a cap-sensitive, flexible, self-supporting explosive charge; a thin-walled tubular member in fixed contact with the charge; and means for coaxially positioning a blasting cap in the tubular member so that the caps base charge is in initiating relationship with the self-supporting explosive charge and both ends of the cap are protected from exposure, e.g., to potentially hazardous conditions. An assembly comprising the priming device and a blasting cap positioned therein, in which assembly the leg wires or fuse, which are adapted to connect the cap to an outside energy source, are secured to the tubular member so as to prevent the relative movement of cap and tubular member, and thereby maintain the initiating relationship between the caps base charge and the selfsupporting explosive charge.

7 Claims, 3 Drawing Figures PAIENIED JAN 2 9 ES? FIG- FBG-

FIG-

EXPLOSIVE PRIMER BACKGROUND OF THE INVENTION This invention relates to a means for initiating the detonation of explosive charges, and in particular to a priming device which is especially adapted for use in initiating relatively insensitive blasting agents in smalldiameter boreholes.

In recent years there has been a trend toward the use of blasting agents which contain little or no high explosive and depend primarily for explosive strength upon such materials as ammonium nitrate. While these blasting agents, when brought to detonation, are characterized by sufficient strength for most blasting operations, they normally are not detonatable by commercially available blasting caps, and the less sensitive compositions can be initiated only by a primer which detonates at highvelocity with a high detonation pressure. Primers which have been used heretofore to detonate such cap-insensitive blasting agents generally have been relatively large, heavy units, e.g., units weighing about 50 grams or more. Such primers have been found to be unsatisfactory not only because of their large dimensions per se, but also for various other reasons, including a relatively high cost of manufacture, less than the desired resistance to abuse, undesirable explosive properties such as impact sensitivity, etc. When primers are needed for blasting operations employing smalldiameter boreholes, such as are employed in underground operations, where the diameter of the borehole may be less than 2 inches, for example, compactness and efficient use of the primer explosive composition are especially important. Reliability, i.e., assurance that the primer explosive will detonate in small diameters at sufficiently high velocity to develop sufficient pressure to cause the blasting agent to detonate at its expected velocity, and safety of design are primary requisites.

SUMMARY OF THE INVENTION This invention provides, as a new article of manufacture, a priming device for use in initiating the detonation of blasting agents, which device, when suitably assembled with an electric or nonelectric blasting cap, performs reliably and safely in initiating cap-insensitive blasting agents in small-diameter, e.g., less than 3 inch, boreholes.

The priming device comprises (a) a cap-sensitive, flexible, self-supporting explosive charge, e.g., in the form of a cylinder; (b) a thin-walled tubular member, e.g., a paper tube, in fixed contact with the explosive charge, preferably disposed within an axial perforation in a cylindrical explosive charge, and adapted to receive a coaxially positioned blasting cap, the tubular member having one end which extends beyond the boundary plane of the charge; (c) means, e.g., a wire, communicating with the interior of the tubular member and adapted to engage the blasting cap so that the base charge thereof is positionedin initiating relationship with said explosive charge; and ((1) means to prevent the relative movement of the blasting cap and the tubular member.

This invention also provides a new assembly for initiating the detonation of blasting agents comprising: (a) a cap-sensitive, flexible, self-supporting explosive charge; (h) a thin-walled tubular member in fixed contact with the explosive charge; (c) a blasting cap,

e.g., an electric or nonelectric cap, positioned coaxially within the tubular member, the tubular member having one end which extends beyond the actuation end of the blasting cap, and this tubular member end and the cap actuation end both extending beyond the boundary plane of the charge; (d) means communicating with the interior of the tubular member and engaging the blasting cap so as to position the caps base charge in initiating relationship with said explosive charge; and (e) means to prevent the relative movement of the blasting cap and the tubular member, e.g., by securing to the tubular member the actuation-energy-transmitting means adapted to lead from the blasting cap to an outside energy source.

BRIEF DESCRIPTION OF THE DRAWING In the accompanying drawing, which illustrates specific embodiments of this invention:

FIG. 1 is a longitudinal cross-sectional view of a priming device of the invention;

FIG. 2 is a side plan view in partial cross section of an initiating assembly of the invention which includes the priming device shown in FIG. 1 and an electric blasting cap seated therein; and

FIG. 3 is a top plan view showing the peripheral configuration of a priming device shown in FIG. 1.

In the drawing, like numerals designate similar parts.

DETAILED DESCRIPTION OF THE INVENTION Although the primer of this invention can be pro vided in any size desired, an outstanding feature of the device is that it can function satisfactorily with small size primer explosive charges, e.g., cylindrical charges weighing as little as about 1 to 2 grams, and measuring as little 0.3 inch in length and one-half inch in diameter. Consequently, any standard blasting cap to be used to initiate the explosive charge in .a primer of such size will be longer than the explosive charge and not be capable of being contained within the charge itself. Exposure of the ends of the blasting cap may present potential safety hazards in use, e.g., as a result of abrasion, shock, impact, or violent ejection of particles, such as ammonium nitrate prills, against the ends of the cap during borehole loading. The present primer contains, in addition to the explosive charge, a thin-walled tubular member, e.g., a paper tube, in fixed contact with the explosive charge, and a blasting-cap-positioning means, e.g., a wire, communicating with the interior of the tubular member. The latter extends beyond the boundary of the charge at least at one end for a sufficient distance that the actuation end of the blasting cap, which is to be coaxially positioned in the tubular member, is protected by the surrounding tubular wall. The actuation end of the blasting cap is the end which houses the ignition element to which a pair of leg wires or safety fuse is attached. The positioning means assures that the base charge of the cap is in initiating relationship with the charge of primer explosive and therefore that the bottom end of the cap also is protected from exposure.

The tubular member, which protects the blasting cap, also serves as a means of anchoring the leg wires or safety fuse leading to the cap from an external source, and thereby preventing movement of the cap relative to the tubular member after cap positioning.

Referring now to the primer shown in FIG. 1, which, for such reasons as ease of manufacture and ruggedness, is a preferred primer, 1 is a cap-sensitive, flexible, self-supporting explosive charge in the form of a cylinder having an axial perforation 2 extending from boundary la to boundary 1b of charge 1. Tubular member 3, e.g., a paper tube having a thin (about 0.02 inch, for example) wall, fits snugly in perforation 2, extending beyond boundary la and boundary lb. Fastening means 4, e.g., a staple made of fine-gage (for example, about 18 gage) wire, passes through charge 1 and tubular member 3 with its crown parallel to the longitudinal axis of tubular member 3, thereby serving to hold charge 1 and tubular member 3 in fixed contact with each other. The wire staple which constitutes fastening means 4 has a leg 40 which is a positioning means for a blasting cap which is to be coaxially positioned in tubular member 3. The length and the position of tubular member 3 can vary, but will always be such that one end thereof will extend beyond (in a direction away from boundary la) the actuation end of a blasting cap plosive charge 1 is scalloped, which configuration may be desirable, e.g., for purposes of ease of handling, and may afford an enhancement of detonation pressure.

In the primer/blasting cap assembly depicted in FIG. 2, electric blasting cap 5 is in coaxial position in tubular member 3 with its bottom end resting against positioning means 4a. With cap 5 in this position, the base charge of load of high explosive in the bottom of the cap shell is in initiating relationship with charge 1. Thus, positioning means 4a serves as a detent means to prevent cap 5 from passing so far through tubular member 3 that no portion of the base charge in the cap is located within the confines of charge 1, i.e., within the planes of boundaries la and lb, a condition which would preclude reliable initiation of charge 1. Of course, the positioning means also must allow the bottom of the blasting cap to at least reach the plane of boundary 1a of charge 1 to achieve the required initiating relationship.

Blasting cap 5 which can be any known type of electric blasting cap capable of initiating explosive charge 1, contains an ignition element attached to a pair of insulated leg wires 6 and 7, which pass through the extended portion of tubular member 3 and exit from the end of this portion. In order to prevent blasting cap 5 from inadvertently being pulled out ofits initiating relationship position with respect to charge 1, the end of the portion of tubular member 3 through which the leg wires pass is folded back and the exiting wires are wrapped around tubular member 3 and anchored with a half hitch.

The active component of the primer of this invention is a flexible, self-supportingexplosive charge which is cap-sensitive, i.e., which detonates consistently when initiated by a standard No. 6 blasting cap. The selfsupporting feature is an'economic advantage in. that packaging expenses are avoided, and flexibility is advantageous from the point of view of ease of handling and assembling and also because the possibility of cracking or chipping upon dropping, as tends to occur with brittle materials, such as compacted or cast bodies, is greatly reduced. The explosive charge also has low sensitivity to impact, and is not deleteriously affected by long periods of immersion in water. The

charge can be made up of multiple components, but

preferably, on an economic basis, is a unitary charge.

The above property requirements are met in compositions comprising a cap-sensitive crystalline high explosive, e.g., pentaerythritol tetranitrate (PETN), admixed with a binding agent, such as are described in U.S. Pat. Nos. 2,992,087, 2,999,743 and 3,428,502, the disclosures of which are incorporated herein by reference. Any of these compositions can be employed for producing the explosive charge of the present primer, and the particular selection of variables, such as the specific components and the quantity thereof employed, particle size of crystalline components, and amount and type of plasticizer used, if any, will depend, inter alia, on the specific explosive properties desired. A preferred composition is one made by mixing finely divided PETN, soluble nitrocellulose having a degree of polymerization of 2,000-3,000, and a plasticizing component such as tributyl 2-acetoxy-l,2,3- propanetricarboxylate as described in U.S. Pat. No. 2,992,087.

The configuration of the self-supporting explosive charge is not critical to the operation of the primer, and compositions such as those described above can be formed into any desired shape by such well-known means as extruding, molding, or rolling. A generally cylindrical configuration, e.g., a circular cylinder having a smooth outer wall or a scalloped or serrated outer wall as shown in FIG. 3, is preferred on the basis of ease of manufacture and use. Angular blocks or disks can also be used, however. Surfaces such as the top and bottom surfaces of cylinders or disks can be flat or curved.

As stated previously, the primer of the invention is used to its best advantage in small boreholes where the overall size of the primer is desirably small, and its weight low. The explosive charge in the primer weighs at least about I gram, and preferably at least about 4 to 6 grams, which weight affords an extra measure of reliability of initiation. Naturally, much heavier charges also can be used. The length of the charge, or the dimension parallel to the axis of the tubular member in contact therewith, usually will be in the range of about from 0.30 to 2.50 inches. The diameter of the charge, or the dimension normal to the tubular axis, usually will be in the range of about from 0.50 to 1.00 inch.

While the detonation velocity of the charge is not critical, as the charge dimensions are smaller it becomes more important to use explosive compositions detonating at higher velocities, e.g., at about 7,000 meters per second or hither.

The thin-walled tubular member can be positioned in fixed contact with the self-supporting explosive charge in various ways, including insertion in a partial bore in the charge, insertion through a full bore in the charge, attachment to an outer wall of the charge (with the tubular outer wall in contact with a charge surface, or with an end of the tubular member in contact with a charge surface), or by having the tubular member constitute an extension of a charge having a thin outer housing. For reasons of ease of production and handling and versatility of design, passage of the tubular member through a full perforation in the charge is preferred. While the location and attitude of the perforation in the charge is not critical to the performance of the primer, i.e., the perforation can be axial, nonaxial but parallel to a major charge axis (e.g., a cylindrical axis), or nonparallel to such axis, a parallel perforation, and usually an axial perforation, is preferred, as this is most convenient to provide, especially in small charges.

The inner diameter of the tubular member must be sufficiently large to allow the insertion therein of the blasting cap to be employed in the primer/blasting cap assembly, e.g., a tubular inner diameter of about 0.30 inch. Since the tubular member constitutes a protective holding means for the cap, the length of the tubular member is such that the tubular member, alone or in conjunction with the wall of a perforated charge, extends beyond both ends of the blasting cap, thereby protecting the ,cap ends from exposure to potentially hazardous conditions. In the case of a perforated charge as shown in the accompanying drawing, the tubular member need not extend through the entire perforation and beyond, but the lower portion of the cap can be protected by the charge itself, while the tube protects the remainder of the cap. However, a more stable structure is achieved by having the tubular member pass at least through the entire perforation, and this constitutes a preferred embodiment. The latter embodiment is also advantageous in that it makes it more practical to fasten the charge and the tubular member together by pinning. The preferred primer contains a tubular member which can accommodate blasting caps of all lengths, and also is long enough that its end near the actuating end of the cap can be folded back, if desired, to assist in anchoring of cap leg wires or safety fuse. A length of about 8 inches meets these requirements.

In primers in which the tubular member passes through a perforation in the explosive charge, one end of the tubular member extends beyond the plane of the boundary ofthe charge which is closest to the actuation end of the cap, i.e., boundary 1a in FIGS. l and 2, and this extension usually will constitute the major length of the tubular member. Extension of the latter beyond the plane of the opposite boundary of the charge, i.e., boundary lb in FIGS. 1 and 2, is not required, but a short extension, e.g., of about one inch, is desirable so as to provide additional protection to the bottom of the cap which is within the confines of the charge, and to provide space around the primer for better contact with the blasting agent in the bottom of a borehole. Although the tubular member can extend beyond the charge boundary plane near the bottom ofthe cap, and thus protect this end of the cap, it is preferred that the bottom of the cap not extend beyond this plane, inasmuch as this would render it difficult to maintain the initiating relationship between the self-supporting explosive charge and the base charge in the cap.

The tubular member has a certain degree of rigidity so that the blasting cap ends are sufficiently protected, e.g., from impact, but a certain degree of deformability also is required if the end of the member is to be folded back, as shown in FIG. 2. As thin a tubular wall as is practical is preferred to assure propagation of a sufficiently strong shock across the tube wall from the cap base charge to the self-supporting explosive charge. Any material of construction which has sufficient stability in the intended borehole environment can be used, e.g., paper or plastic. A paper (waxed or unwaxed) or plastic tube about mils thick is satisfactory for most conditions.

The means by which the tubular member and selfsupporting explosive charge are held together in fixed contact can be mechanical, e.g., wires, pins, a staple, etc., or the adjacent surfaces can be held together by means of an adhesive. A mechanical fastening means is easy to apply to the joined components, and can also serve, totally or in part, as the positioning means for the blasting cap. Therefore, such a fastening means is preferred.

The means for positioning the blasting cap, i.e., locating the cap so that its base charge is in initiating relationship with the self-supporting explosive charge, can be any means which communicates with the interior of the tubular member and engages the cap so as to allow it to be moved to its preselected position and no farther. The positioning means can be, for example, a constriction in the tubular member wall, a collar, or a member which forms a barrier spanning the tubular member, e.g., a wire, pin, or plug, against which the bottom end of the cap is to be seated. A preferred positioning means is a leg of a wire staple, as indicated by 4a in the accompanying drawing, the staple serving at the same time as the fastening means for the selfsupporting charge and tubular member. The barrier usually will constitute an inert component of the primer, and preferably will be of minimum dimensions to maximize the primer efficiency. For example, wires or pins preferably are no thicker than about 0.10 inch.

In the primer/blasting cap initiating assembly of this invention, the relative movement of the blasting cap and the tubular member, which could lead to a breakdown in the initiating relationship between the base charge of the cap and the explosive of the primer, is prevented by securing the leg wires or safety fuse which will connect the blasting cap to an external energy source, to the tubular member which protects the blasting cap. In the case of an electrical blasting cap, for ex ample, the leg wires which emerge from the end ofthe tubular member may be tied around the tubular member at the emerging end or the end opposite thereto. Preferably, the end of the tubular member and the wires are folded back, e.g., as shown in FIG. 2, and the wires tied. Alternatively, the wires may be led out through one or two passages in the tubular wall and tied. In the case of a fuse-actuated cap, the sefety fuse in the primer can be kept from moving by folding the end of the tubular member and fuse, tying being unnecessary. Of course, other means of securing the wires or fuse to the tubular member, e.g, adhesive tape, also can be used. I

The present primer can be used with any type of blasting cap, electric or nonelectric, instantaneous or delay. The assembly is readily put together in the field by inserting the cap, bottom first through the long end of the tubular member until the bottom is seated against the positioning means; and securing the wires or fuse to the primers tubular member.

The following examples serve to illustrate specific EXAMPLE 1 Primer Preparation A flexible, self-supporting explosive composition was made by mixing PETN, nitrocellulose, and tributyl 2- acetoxy-l,2,3-propanetricarboxylate according to the teachings of the examples in the aforementioned US. Pat. No. 2,992,087, issued on July 11, 1961, to H. H. Fassnacht, R. W. Felch, and C. D. Forrest. The PETN content of the composition was 72 percent, and the size of the PETN particles and the type of nitrocellulose used were those described in the examples of the specified patent. The composition was extruded through a die which formed it into a tube having a circular internal configuration and a rosette-shaped or scalloped outer surface consisting of six equal circular arcs. The major outer diameter of the tube was 0.690 inch, the minor outer diameter 0.530 inch, and the internal diameter 0.330 inch. The tube was cut into l-fls-inch lengths, each cut section'forming an explosive charge weighing about 6 grams. These charges had the sensitiveness to initiation by a blasting cap, thermal stability, insensitiveness to impact, water resistance, flexibility, and other properties described for such a formulation in the cited US. Pat. No. 2,992,087.

An S-inch-long tube constructed of waxed Kraft paper and having an inner diameter of 0.300 inch and a 0.020-inch thick wall, was inserted through the perforation in each charge of flexible explosive in a manner such that one end of the charge was located one inch from one end of the paper tube. An aluminum staple having 9/] 6-inch-long legs and a 3/l6-inch-long crown was applied to the paper tube/explosive charge assembly, the legs of the staple passing through the charge and the tube wall, and the crown of the staple being parallel to the longitudinal axis of the tube. The end of the explosive charge which was 1-inch distant from the end of the paper tube was 3/l6-inch distant from the nearest staple leg. Preparation of Primer/Cap Assembly Delay electric blastic caps providing delay periods required for a given blasting round were employed. A cap was inserted, bottom end first, into the end of the paper tube farthest from the explosive charge, and was seated against the first leg of the aluminum staple which it encountered, this staple leg acting as a cap stop or detent means. The caps leg wires led from the cap, through the tube, and out the end of the tube farthest from the charge. The paper tube and included cap leg wires were bent back 180 at a location about onehalfinch from the end of the tube, and a half hitch was made with the wires around the tube where they emerged from the bent-over end of the tube. Evaluation When these assemblies were tested for their ability to initiate an ammonium nitrate/fuel oil (ANFO) composition containing 5.2 percent fuel oil, in 2-inch diameter pipe, the ANFO was initiated reliably and detonated at velocities comparable to those achieved when standard dynamite primer sticks about 1 inch X 4 inches in size and weighing about 50 grams were used.

, in field trials, the results obtained in rock blastin with 94/6 (percent) AN/fuel oil primed with the primer/cap assembly described above (placed in the holes with the bottom of the cap toward the bottom of thehole) in l and l-%-inch diameter holes, were comparable to those achievedwith heavier and bulkier dynamite primers (1- /1 X 8 inch and l-% X 16 inch dynamite).

The lower sensitivity to impact of the primer/cap assembly of this invention was contrasted to that of a blasting cap which is not protected as is the cap in the present assembly, was shown by drop weight tests.

EXAMPLE 2 The procedure described in Example 1 for the preparation of a primer/cap assembly was repeated with the exception that nonelectric blasting caps with safety fuse inserted therein adjacent the ignition charge were substituted for the electric blasting caps. In the assembly, the fuse led from the cap, through the tube, and out the end of the tube, as did the wires in Example 1. In this case, however, the paper tube and included fuse were bent back as described in Example 1, and this procedure alone held the cap in place with respect to the tube. The assembly was placed in the holes with the tube bent toward the bottom of the hole. The results obtained in field trials with these assemblies were comparable to those achieved with the primer/cap assemblies described in Example 1.

I claim:

1. An assembly for initiating the detonation of blasting agents comprising:

a. a cap-sensitive, flexible, self-supporting explosive charge;

b. a thin-walled tubular member in fixed contact with said explosive charge;

c. a blasting cap provided with means adapted to transmit actuation energy thereto from an outside source, said blasting cap being positioned coaxially within said tubular member, said tubular member having one end which extends beyond the actuaion end of said blasting cap, and said tubular member end and said cap actuation end both extending beyond the plane of the boundary of said explosive charge;

d. means communicating with the interior of said tubular member and engaging said blasting cap so as to position the base charge thereof in initiating relationship with said explosive charge; and

e. means comprising the exterior of said tubular member in cooperative relationship with said means adapted to transmit actuation energy to said blasting cap to prevent the relative movement of said blasting cap and said tubular member.

2. The assembly of claim 1 wherein said explosive charge is in the form of a cylinder having an axial perforation extending throughout its length, and said tubular member is positioned within said perforation.

3. The assembly of claim 1 wherein said tubular member is of paper-based construction, and said means engaging said blasting cap is also a means which holds said explosive charge and said tubular member in fixed contact.

4. The assembly of claim 3 wherein said means engaging said blasting cap is a wire which passes through said explosive charge and said tubular member normal to the tubular axis, and the bottom end of said blasting cap is seated against said wire.

5. The assembly of claim 4 wherein said wire is a leg of a staple.

6. The assembly of claim 3 wherein said blasting capis an electrically actuated cap and said means adapted to transmit actuation energy to said blasting cap are the leg wires of said cap, said leg wires passing through the portion of said tubular member extending beyond said plane, emerging from the end of said tubular member, and being bent together with said extended portion, the

fuse in said cap, said fuse passing; through the portion of said tubular member extending beyond said plane, emerging from the end of said tubular member, and

being bent together with said extended portion. 

1. An assembly for initiating the detonation of blasting agents comprising: a. a cap-sensitive, flexible, self-supporting explosive charge; b. a thin-walled tubular member in fixed contact with said explosive charge; c. a blasting cap provided with means adapted to transmit actuation energy thereto from an outside source, said blasting cap being positioned coaxially within said tubular member, said tubular member having one end which extends beyond the actuaion end of said blasting cap, and said tubular member end and said cap actuation end both extending beyond the plane of the boundary of said explosive charge; d. means communicating with the interior of said tubular member and engaging said blasting cap so as to position the base charge thereof in initiating relationship with said explosive charge; and e. means comprising the exterior of said tubular member in cooperative relationship with said means adapted to transmit actuation energy to said blasting cap to prevent the relative movement of said blasting cap and said tubular member.
 2. The assembly of claim 1 wherein said explosive charge is in the form of a cylinder having an axial perforation extending throughout its length, and Said tubular member is positioned within said perforation.
 3. The assembly of claim 1 wherein said tubular member is of paper-based construction, and said means engaging said blasting cap is also a means which holds said explosive charge and said tubular member in fixed contact.
 4. The assembly of claim 3 wherein said means engaging said blasting cap is a wire which passes through said explosive charge and said tubular member normal to the tubular axis, and the bottom end of said blasting cap is seated against said wire.
 5. The assembly of claim 4 wherein said wire is a leg of a staple.
 6. The assembly of claim 3 wherein said blasting cap is an electrically actuated cap and said means adapted to transmit actuation energy to said blasting cap are the leg wires of said cap, said leg wires passing through the portion of said tubular member extending beyond said plane, emerging from the end of said tubular member, and being bent together with said extended portion, the portions of the leg wires protruding from the end of said tubular member being tied around said tubular member.
 7. The assembly of claim 3 wherein said blasting cap is a safety-fuse-actuated cap and said means adapted to transmit actuation energy to said blasting cap is the fuse in said cap, said fuse passing through the portion of said tubular member extending beyond said plane, emerging from the end of said tubular member, and being bent together with said extended portion. 